1. Field of the Invention
The invention relates to a method of attitude control for spacecraft with flexible structures, particularly to compensate for disturbances caused typically by offset between the center of pressure of spacecraft thrusters and the spacecraft center of mass.
2. Description of Related Art
An important function of a spacecraft attitude control system utilizing thrusters is compensation for thrusting disturbances about each control axis. These disturbances are primarily induced by offsets of the spacecraft's center of mass from the thrusters' center of pressure as well as thruster imperfections. Additional causes for thrusting disturbances are changes in the spacecraft's center of mass over time or possible thruster failure. These changes, which are impossible to accurately calculate in advance, render a prebias approach to attitude compensation, wherein corrections for thrusting disturbances are precalculated from known spacecraft properties and then fed in to the thruster command processors, quite difficult. The prior art has relied on either such a prebias approach or a high bandwidth control loop to compensate for the bias.
Chan, in U.S. Pat. No. 4,537,375, relies on a servo loop incorporating a prebias at the input end of the thruster modulation logic. The calculated prebias values are derived from measured values of thrusting disturbance obtained during calibration maneuvers and stored either in spacecraft memory or on the ground. Importantly, the gains in Chan's servo loop system are constant, and he ideally requires orthogonal thrusters arranged in couplets, with each thruster in the couplet being able to counteract the torque produced by the other. Furthermore, Chan's approach also requires calibration maneuvers from time to time as spacecraft mass properties change in order to obtain the values for the prebias to be stored in memory or transmitted to ground control for later use.
Bittner, in U.S. Pat. No. 4,567,564, eliminates this prebias approach and its consequent need for calibration, by using a high bandwidth observer/state controller combination for disturbance torque compensation. While this approach obviates the need for calibration maneuvers or ground input during disturbance torque compensation, Bittner's method increases the difficulty of control system design by requiring a high-enough fixed gain to converge state variables rapidly before attitude error exceeds the pointing requirement while simultaneously stabilizing spacecraft structural modes. In some systems these goals cannot be simultaneously accomplished.
Chan, in U.S. Pat. No. 4,767,084, relies on momentum/reaction wheels for attitude control during stationkeeping and thrusters for desaturation of the momentum/reaction wheels during East/West drift compensation maneuvers. This technique, however, requires a low-bandwidth wheel control loop to stabilize structural modes, thereby degrading spacecraft pointing performance. The length of Chan's desaturation maneuvers is predetermined by an autodesaturation logic which alternately fires East and West thrusters after East/West drift has been reduced to zero.